The invention relates to a mold closing device for a plastics molding machine of the type including a machine frame, two fixed plates supported by the machine frame and facing each other in parallel relationship at a distance therebetween, at least one moveable mold clamping plate moveably guided in the frame, at least one closing drive supported on one of the fixed plates and capable of applying a closing force to the moveable mold clamping plate wherein the force is directed parallel to a closing force axis in the direction of the other fixed plate, and four tension members having identical shapes and dimensions and interconnecting the two fixed plates in a tension-resistant manner and absorbing the reaction force exerted by the closing drive on the two fixed plates, wherein each fixed plate is connected to the tension members at two upper attack points and two lower attack points which are located above and below, respectively, of the horizontal plane extending through the closing force axis.
The invention is preferably applicable to an injection molding machine, wherein in this case only a moveable mold clamping plate is provided and wherein one of the fixed plates is formed as a stationary mold clamping plate. The other fixed plate is the support plate of the closing drive. The invention, however, can also be used with mold closing devices provided with two moveable mold clamping plates which can be moved against each other by two closing drives. Both fixed plates are in this case support plates for the two closing drives. This construction is found especially with blow molding machines.
The force exerted by the closing device for closing the form acts on the two fixed plates as a reaction force pressing the plates apart. In addition, the plastic injected into the closed form exerts a driving force pressing the form halves apart, which force also has to be absorbed by the fixed plates. Consequently, the two fixed plates have to be connected by adequately sized tension members for preventing the fixed plates from moving apart and thus the form from opening under the injection pressure.
The tension members of injection molding machines of conventional design are constructed of four straight rods or columns which interconnect the two fixed plates, i.e. normally the stationary mold clamping plate and the support plate supporting the closing cylinder, wherein the tension members are connected in the corner regions of these plates. In this way, a reliable mutual support of the fixed plates with uniform force distribution over the area of these plates is attained which can reliably prevent the form from opening under the injection pressure, even if the injection pressure is applied non-symmetrically with respect to the center axis of the form as a result of the shape of the mold. The use of straight rods or columns as tension members is disadvantageous because the two rods distal from the frame in particular impede free access to the space between the fixed plates. This in particular complicates the use of a handling device intended to be inserted between the open mold halves for removing the finished articles.
The term "fixed plate" used above is meant to indicate that the plates are essentially stationary during operation and do not move when the form tools open and close. Normally, however, at least one of these fixed plates, possibly also both, are supported on the machine frame for limited movement in such a way that they can under the influence of the closure force execute small longitudinal displacements in the longitudinal direction in order to compensate for changes in length of the tension members.
In order to eliminate an obstruction of the access to the space between the forms, so-called rod-less injection molding machines (i.e. injection molding machines without cross-beam) were proposed. Instead of the straight rods or columns on each side of the machine, these machines have a C-shaped tension member which interconnects the two fixed plates with each other and extends in the region between the fixed plates essentially below the upper edge of the machine frame, thereby providing access to the space between the fixed plates. Such a rod-less injection molding machine is, for example, known from EP 0 554 068. Utilization of the same principle with C-shaped tension members in a plastics molding machine, in particular for a blow molding machine, with two C-shaped clamping plates which are moveable with respect to each other, is known from U.S. Pat. No. 3,787,165. It is also known from DE 44 11 649 A1 to construct such C-shaped tension members out of several parts, for example, out of two L-shaped parts connected to each other. That does not alter the fact that the L-shaped parts taken together form a C-shaped member which is connected to the fixed plates only at two points.
It is also known to completely eliminate the use of separate tension members and to support the two fixed plates directly on the suitably constructed C-shaped machine frame as described, for example, in DE-U-92 12 480.1.
However, introduction of the closing reaction forces into a C-shaped tension member or a C-shaped machine frame has the consequence that the unavoidable deformation of the C-member is associated with a spreading apart of the C-leg, i.e. with a change in angle of the C-leg. In order to prevent a corresponding change in angle between the fixed plates, i.e. to a loss of parallelism between the fixed plates, all known designs have in common that the two fixed plates are connected at their respective center and tiltable about a horizontal axis with the C-shaped tension members or with the machine frame, respectively. This then allows the two fixed plates to remain parallel with respect to each other independent of the deformations of the C-shaped tension members or of the machine frame. Because the two fixed plates and consequently also the molding tools connected thereto can be tilted about the horizontal axis, the fixed plates and the molding tools can be tilted about this horizontal axis by swelling forces acting eccentrically with respect to the center axis of the molding tool, so that the gap between the molding tools opens on one side, which can result in an release of the plastic melt into the gap and to the formation of a ridge on the injection-molded part. With conventional designs, the advantage of unobstructed access to the space between fixed plates is attained at the expense of increased susceptibility of the device to one-sided pressure of the form tools.